SEOUL » A U.S. ambassador struggled with pain as he recovered Friday from a knife attack, while police searched the offices of the anti-U.S. activist who they say slashed the envoy while screaming demands for Korean reunification.

The attack Thursday on Mark Lippert, which prompted rival North Korea to gloat about "knife slashes of justice," left deep gashes and damaged tendons and nerves. It also raised questions about security in a city normally seen as ultra-safe, despite regular threats of war from Pyongyang.

While an extreme example, the attack is the latest act of political violence in a deeply divided country where some protesters portray their causes as matters of life and death.

Lippert, 42, was recovering well but still complaining of pain in the wound on his left wrist and a finger where doctors repaired nerve damage, Severance Hospital official Yoon Do-Heum said in televised briefing. Doctors will remove the 80 stiches on Lippert’s face on Monday or Tuesday and expect him to be out of the hospital by Tuesday or Wednesday. Hospital officials say he may experience sensory problems in his left hand for several months.

Police, meanwhile, searched the offices of the suspect, Kim Ki-jong, 55, for documents and computer files as they investigated how the attack was planned and whether others were involved. Police plan to soon request a warrant for Kim’s formal arrest and said potential charges include attempted murder.

Kim, who has a long history of anti-U.S. protests, said he acted alone. He has told police he attacked Lippert to protest annual U.S.-South Korean military drills that started Monday — exercises that the North has long maintained are preparations for an invasion. Kim said the drills, which Seoul and Washington say are purely defensive, ruined efforts for reconciliation between the two Koreas, according to police officials.

While most South Koreans look at the U.S. presence favorably, America infuriates some leftist South Koreans because of its role in Korea’s turbulent modern history.

Washington, which backed the South during the 1950-53 Korean War against the communist North, still stations nearly 30,000 troops here and holds annual military drills with Seoul. That’s something anti-U.S. activists view as a major obstacle to their goal of an eventual reunification of the rival Koreas.

"South and North Korea should be reunified," Kim shouted as he slashed Lippert with a 10-inch knife, police and witnesses said.

U.S. Secretary of State John Kerry, in Saudi Arabia for meetings with regional leaders, said the attack would not reduce America’s resolve in pursuing its interests.

"The United States of America will never be intimidated or deterred by threats or by anybody who harms any American diplomats," he said.

Kim is well-known among police and activists as one of a hard-core group of protesters willing to use violence to highlight their causes. Such protesters often speak of their actions in terms of a war, of a struggle to the death.

Police didn’t consider the possibility that Kim, who has ties to the Korean Council for Reconciliation and Cooperation, which hosted the breakfast meeting where Lippert was attacked, would show up for the event, according to a Seoul police official who didn’t want to be named, citing office rules.

U.S. ambassadors have security details, but their size largely depends on the threat level of the post. Seoul is not considered to be a particularly high threat post despite its proximity to the North Korean border. It’s not clear how many guards Lippert had, but they would have been fewer than the ambassadors in most of the Mideast.

Seoul’s Foreign Ministry said it was the first time a foreign ambassador stationed in modern South Korea had been injured in a violent attack.

However, the Japanese ambassador narrowly escaped injury in 2010 when Kim threw a piece of concrete at him, according to police. Kim, who was protesting Japan’s claim to small disputed islands that are occupied by South Korea, hit the ambassador’s secretary instead, media reports said, and was sentenced to a three-year suspended prison term over the attack.

The website of the Woorimadang activist group that Kim heads describes the group’s long history of anti-U.S. protests. Photos show him and other activists rallying last week in front of the U.S. Embassy in Seoul to protest the U.S.-South Korean military drills, which are to run until the end of April.

South Korea’s Unification Ministry says Kim visited North Korea with a civic group eight times between 2006 and 2007, during a period of inter-Korean cooperation under a liberal government in Seoul.

North Korea’s state-controlled media crowed Thursday that Kim’s "knife slashes of justice" were "a deserved punishment on war maniac U.S." and reflected the South Korean people’s protests against the U.S. for driving the Korean Peninsula to the brink of war because of the joint military drills.

Activists in Seoul expressed worries that the attack on Lippert would harm the public image of peaceful protesters, or prompt the conservative government to suppress their activities.

Small to medium-sized demonstrations regularly occur across Seoul, and most are peaceful.

But scuffles with police do break out occasionally, and the burning of effigies of North Korean and Japanese leaders is also common. Some demonstrators have also previously severed their own fingers, thrown bodily fluids at embassies and tried to self-immolate.

Lippert became ambassador last October and has been a regular presence on social media and in speeches and presentations during his time in Seoul. He’s regularly seen walking his Basset Hound, Grigsby, near his residence, not far from where the attack happened. His wife gave birth here and the couple gave their son a Korean middle name.

Associated Press writers Hyung-jin Kim in Seoul and Matthew Lee in Riyadh, Saudi Arabia, contributed to this report.